In optical data transmission, digital data values may be transmitted by means of an optical transmission signal. The optical transmission signal is generated, by modulating the phase and/or the amplitude of an optical carrier signal, which possesses a carrier frequency, in dependence on the transmitted data values and in accordance with a constellation diagram of a respective phase-shift keying (PSK) modulation or Quadrature Amplitude Modulation (QAM) method. Each point of the constellation diagram represents a finite set of data values that are to be transmitted, wherein the set of data values is called a data symbol. A data symbol is represented by a corresponding constellation point of a constellation diagram, wherein the constellation point has a corresponding symbol phase and amplitude value. Depending on the data symbols that are to be transmitted, respective constellation points and symbol phase values are derived. The phase and the amplitude of the optical carrier signal are modulated, such that it corresponds to the derived symbol phase values and amplitude values representing the respective data symbols.
When using a pure PSK modulation method, only the phase is modulated, while the amplitude is kept constant. When using a QAM method, the phase is modulated just as in a PSK scheme, but also the amplitude is modulated for obtaining a higher data rate. Thus, a QAM method may be interpreted as a PSK modulation method with an additional amplitude modulation. Therefore, in this patent application, the term PSK modulation method shall include a pure PSK modulation method as well as a QAM method.
Preferably, the data values are data bits. An example for a phase-shift keying modulation method is Binary Phase-Shift Keying (BPSK), in which each point of the corresponding constellation diagram represents one bit and in which adjacent constellation points are separated by a separation angle that has an absolute value of π. Another example of a phase-shift keying modulation method is Quadrature Phase-Shift Keying (QPSK), in which each constellation point represents a data symbol consisting of two bits and in which adjacent constellation points are separated by a separation angle that has an absolute value of π/2.
At a receiving side, received data values may be derived, by carrying out a coherent reception scheme: the received optical transmission signal is mixed with a coherent optical signal that possesses the carrier frequency and a phase, which is ideally equal to the phase of the optical carrier signal used at the transmitting side. This mixing yields a resulting optical baseband signal. The optical baseband signal is converted to a sampled electrical signal via analogue-digital conversion, and the phase of the sampled electrical signal is estimated for deriving received data values. When using a hard decision detection scheme, it is decided for that point of the constellation diagram, whose symbol phase value is most similar to the estimated phase of the received optical carrier signal. Corresponding data symbols and corresponding data valued are then derived from the estimated symbol phase values.
When transmitting an optical signal over an optical transmission channel, the signal's phase may be degraded by a phase offset that may be estimated at the receiver. The phase of the PSK modulated optical signal may then be corrected by this estimated phase offset. The estimated phase offset may be erroneous to a degree, such that the correction of the received optical signal causes a rotation of the PSK constellation diagram by a whole numbered multiple of the separation angle from the receiver's perspective. Such a rotation of the constellation diagram occurs from the time instance of one data symbol to a next time instance of a next successive data symbol and is called a phase slip. A typical value for a probability of a phase slip is for example 10−3.
When relying on a hard decision detection scheme at the receiver, the effects of a phase slip are the following: If a phase slip causes a rotation of the constellation diagram at a certain time instance, and if the constellation diagram maintains to be rotated for further time instances, then this leads to false derived data values for the certain time instance and for the further time instances.
A known countermeasure against phase slips is the technique of differential coding. On the transmission side, the derived data symbols are differentially encoded into differentially encoded data symbols, such that a derived data symbol is represented by a transition from one differentially encoded data symbol to a next successive differentially encoded data symbol. The differentially encoded data symbols are then mapped onto the PSK constellation diagram. On the receiving side, the received differentially encoded data symbols are observed. Differential decoding may be performed, by deriving a differentially decoded data symbol from a transition from one differentially encoded data symbol to a next differentially encoded data symbol. In other words, data symbols are derived at the receiver by phase changes between successive received differentially encoded data symbols. The derived differential decoded data symbols represent differential decoded data values, as previously mentioned.
When relying on differential encoding in combination with a hard decision detection scheme at the receiver, the effects of a phase slip are the following: If a phase slip causes a rotation of the constellation diagram at a certain time instance, and if the rotation of the constellation diagram prevails for next further time instances, then this leads to false derived data values for the certain time instance but not for the next further time instances; but, if a phase slip causes a rotation of the constellation diagram only for the certain time instance, and if the rotation of the constellation diagram does not prevail for the next further time instances, then this leads to false derived data values for the certain time instance and also for that single next time instance that succeeds the certain time instance.